Collimation device for irradiation apparatus

ABSTRACT

A collimation device for an irradiation apparatus comprising in particular a radioactive source having a generally flat shape constituted by a source-holder frame in which radioactive elements are disposed in spaced relation, characterized in that it comprises a mask having open portions which is placed in parallel relation in front of at least one face of said source and in proximity thereto, said mask being made up of a series of screens formed of absorbent material and adapted to project in the direction of packages to be irradiated which pass in front of said source, each screen having a transverse cross-section which decreases from said source towards said packages.

United States Patent 1 Eymery 51 Jan. 9, 1973 [54] COLLIMATION DEVICEFOR IRRADIATION APPARATUS [75] Inventor:

[73] Assignee: Commissariat A Atomique, Paris, France [22] Filed: Oct.8, 1969 [21] Appl. No.: 864,692

Rene Eymery, Grenoble, France LEnergie [30] Foreign Application PriorityData [58] Field of Search .....250/106 S, 106 R, 105 R, 52; 313/117;350/271, 276; l78/5.4; 219/354; 240/4601, 46.31, 108

[56] References Cited UNITED STATES PATENTS 2,635,203 4/1953 Pakswer..3l3/92 PD Primary Examiner-James W. Lawrence Assistant Examiner-D. C.Nelms Attorney-Craig, Antonelli, Stewart and Hill [57] ABSTRACT Acollimation device for an irradiation apparatus com prising inparticular a radioactive source having a generally flat shapeconstituted by a source-holder frame in which radioactive elements aredisposed in spaced relation, characterized in that it comprises a maskhaving open portions which is placed in parallel relation in front of atleast one face of said source and in proximity thereto, said mask beingmade up of a series of screens formed of absorbent material and adaptedto project in the direction of packages to be irradiated which pass infront of said source, each screen having a transverse cross-sectionwhich decreases from said source towards said packages.

8 Claims, 8 Drawing Figures PATENTEDJAN 9191s saw 1 or 4 RENE: EYMERYATTORNEYS a r 1 l/ I IV I z 4 r PAIENIEDJMI' 9 Ian SHEET 2 [IF 4INVENTOR REA/E EYMERY lcl a w Y M ATTORNEY 3 PATENTEDJAN 9 I975 SHEEI 30F 4 INVENT OR REA/L: EYMERY 1 AITORNEYS COLLIMATION DEVICE FORIRRADIATION APPARATUS This invention relates to a collimation device foran apparatus in which objects or packages are exposed to radiation asthey pass in front of a radioactive source and in which betteruniformity of doses received by said packages at all points is achievedby means of said device.

Different methods or special devices for improving the distribution andhomogeneity of radiation doses delivered to the interior of packages tobe irradiated are already known. Thus, it is possible to increase thenumber of transfers of packages on each side of the radioactive sourcewhich is usually provided in the form of a vertically disposed plaqueand which contains a series of suitably distributed radioactiveelements. This is achieved by means of a conveyor which is designed tomove the packages in front of the source a number of times in parallelrelation to the faces of the source and at a distance from this latterwhich varies from one pass to the next.

A method of the type just mentioned makes it possible in particular toattenuate the heterogeneity of doses received in the transversedirection at right angles to the plane of the source however, thisimprovement is limited inasmuch as the most homogeneous doses deliveredduring passes which are carried out at a greater distance from thesource are also the lowest in value and can correct only to a verypartial extent the principal heterogeneity which results from passescarried out within close proximity to the source. If the initial passesare carried out at an even greater distance away in order to enhance thehomogeneity of radiation doses, the efficiency of the installation fallsoff rapidly and limits the value of the improvement which it wasintended to achieve. Whatever procedure may be adopted, it isnevertheless sought in the majority of cases to limit the thickness ofpackages while retaining the same density of constituent materials inorder to improve the homogeneity in the transverse direction.

In order that the dose received by said packages should also be madeuniform in the vertical direction, a transfer is also carried out infront of the source but at different levels the packages themselvesshould preferably have dimensions such that they extend to anappreciable distance beyond the apparent contour of the source. Finally,another method consists in detere mining in the plane of the source thebest distribution of radioactive elements for the purpose ofregularizing the doses which are delivered. In particular, provision canbe made for a design in which the source projects beyond the top andbottom of the packages to be processed and comprises in this case acentral zone which is relieved of radioactive elements. However, in allthe designs which have been proposed, homogeneity of absorbed doses isnot wholly satisfactory, especially in the case of packages ofappreciable thickness.

The aim of this invention is to circumvent the disadvantages which havebeen discussed in the foregoing by means of a collimation device whichis capable of being placed in proximity to the source so as to limitradiations which are delivered in certain directions while leaving afree passage for these radiations in other directions. Thus, the dosereceived at any point of a package under irradiationv as defined by thesolid angle through which said point sees the source and moreparticularly the radioactive elements considered together whichconstitute said source is substantially uniform both in the transversedirection and vertical direction. In addition, said device makes itpossible to a certain extent to correct the effects of heterogeneity ofthe source itself.

A further aim of the invention is to provide a simple apparatus whichcan readily be placed in position and does not in any way affect thedesign of the source itself and in particular the distribution ofradioactive elements in this latter.

To this end, the collimation device under consideration which is moreparticularly applicable to a radioactive source having a generally flatshape constituted by a source-holder frame in which radioactive elementsin the form of cylindrical rods or point sources are disposed in spacedrelation, is characterized in that it comprises a mask having openportions which is placed in parallel relation in front of at least oneface of said source and in proximity thereto, said mask being made up ofa series of screens formed of absorbent material and adapted to projectin the direction of packages to be irradiated which pass in front ofsaid source, each screen having a transverse cross-section whichdecreases from said source towards said packages.

Said mask preferably comprises screens which extend in front of each ofthe parallel faces of the sourceholder frame.

By virtue of the foregoing arrangements, the screens of the mask areaccordingly interposed between on the one hand the radioactive elementsof the source and on the other hand predetermined points of the packagesto be irradiated. In particular, as a result of the presence of thescreens, said radioactive elements can only partially be seen or inother words are in direct line of access to the nearest face of thepackage considered only to a partial extent whereas the same elementscan be seen directly by points which are located at the center of saidpackages or at an even greater distance from the plane of the source.The disparity between the doses received at these different points,especially between points of the package face which is located nearestthe source in parallel relation to this latter and points which have adifferent location, is thus considerably reduced under these conditions.Thus, the dose applied to those parts of the package which wouldotherwise have received an excessive dose is accordingly reduced insuitable and especially greater proportions than those parts of the samepackage which would have received lower total doses without thecollimation mask. Accordingly, a substantial improvement in homogeneityof the radiation doses which are delivered is thus achieved simply atthe price of a slight reduction in output of the unit, this improvementbeing more marked as the packages are located closer to the plane of thesource.

As an advantageous feature and in the case in which said sourcecomprises a plurality of radioactive elements having the shape ofcylindrical and. parallel rods mounted in said source-holder frame whichis placed in a vertical position, said screens are constituted bymetallic triangular-base prisms. which are parallel to the rods and onelateral face of which isdisposed parallel to the plane of said source,said prisms being placed at intermediate levels with respect to thelevels of said rods.

On the contrary, in the case in which said source comprises single-pointradioactive elements which are disposed on a uniform lattice andespecially a lattice having a square or rectangular pitch, said screensare constituted by a grid having perforations which are placed in frontof each single-point element and are provided with extended portions inthe form of outwardly flared recesses each having a volume in the shapeof a pyramid or truncated cone.

In addition and in accordance with a further property of the invention,said screens in said mask and said radioactive elements in said sourcecan be oriented in such a manner as to have a given inclination to thedirection of motion of said packages in front of said source. It is thuspossible to contemplate a horizontal array of elements and transfer ofpackages at a predetermined angle of slope or, conversely, to give anorientation to the elements and a direction of transfer which have twoangles of inclination as desired but of different value.

Finally, a number of different alternative forms can be contemplated inthe assembly of the mask and of the source thus, said mask can bedirectly secured to the source-holder frame or be separate from thislatter so as to constitute a container in which said frame is slidablyfitted.

Further properties of the collimation device under consideration willbecome apparent from the following description of a number ofexemplified embodiments which are given by way of indication and not inany sense by way of limitation, reference being made to the accompanyingdrawings, in which FIGS. 1 and 2 are views taken in cross-section and inperspective showing a collimation device for an apparatus in whichpackages are subjected to irradiation as they move in front of aradioactive source FIGS. 3 and 4 are schematic diagrams which serve toset forth the. advantages provided by the collimation device-underconsideration for ensuring uniform radiation doses received at thedifferent points of any package respectively in the transverse directionand in the vertical direction FIG. 5 shows diagrammatically analternative construction which is applied to the embodiment of FIGS. 1and 2 and serves to limit the effects arising from inherentheterogeneity of the source FIG. 6 and FIGS. 7 and 8 are respectively asectional view and top views of other alternative embodiments of thecollimation device as adapted more especially to the case in which thesource comprises a series of single-point radioactive elements.

There is shown at l in FIGS. 1 and 2 a radioactive source which isconstituted in this example of construction by cylindrical rods 2 ofsuitable radioactive elements, said rods being set in parallel relationat a suitable distance from each other and mounted horizontally withinthe interior of a source-holder frame 3, the source being designed underthese conditions in the form of a flat plaque which is placedvertically.

In accordance with the invention, the source 1 is associated with acollimation device which is intended to conceal the source to a partialextent and is constituted. by two masks 4 which are placed parallel tothe two faces of the frame 3 and in proximity to these latter. Thesemasks can be separate from each other or joined along their edges so asto form in this case a kind of narrow container in which the source 1can readily be inserted by sliding. Each mask comprises a support 5 fora series of screens 6 each having the shape of a metallic prism with atriangular base and mounted in the support in parallel relation both toeach other and to the cylindrical rods 2, said prisms being uniformlyspaced over a distance such that they are located at intermediate levelswith respect to the positions occupied by the rods 2 in thesource-holder frame 3. There is also shown in FIGS. 1 and 2 a package 7to be irradiated which is carried by a conveyor (not shown in thedrawings) for the purpose of transferring said package in a given numberof passes in front of both faces of the source 1 which is fitted withthe masks 4.

FIGS. 3 and 4 serve to obtain a better understanding of the mode ofdistribution of doses within the package 7 in the case of a radioactivesource and of collimation masks according to the arrangements which arecontemplated in FIGS. 1 and 2, and especially of the manner in whichdoses are distributed at points of said package which are separated fromeach other either transversely (in the case of FIG. 3) or vertically (inthe case of FIG. 4). In order to simplify the drawings, there are shownin cross-section in these figures only three cylindrical rods 2a, 2b, 2cand two prismatic screens 6a and 6b which are placed between the sourceand the package.

If one considers any two points A and B of the package 7 which arelocated at a given transverse distance from each other, it may beestablished that the doses received at these points which are defined bythe solid angle through which said points see all the rods of the sourceare different at A and at B. In the case of the point A which is locatednearest the source, it is thus observed that this point sees the wholerod 2a, approximately two-thirds of the rod 2b and only a very smallportion of the rod 20 by reason of the presence of the two screens 60and 6b. Similarly, the point B sees approximately three-quarters of therod 2a, the whole of the rod 2b and a portion of the rod 20 which isapproximately equal to two-thirds, namely an overall source volume whichis greater than that seen by the point A but at a greater distance.

It is therefore apparent that the points A and B of the package are notsubjected to uniform radiation doses inasmuch as these doses depend inparticular on the shape of the screens which are placed in front of thesource and more especially on the angle a at the base of these prisms.Moreover, it has been shown in FIG. 1 by means of a dashed-linerepresentation that a point located on that face of the package which islocated nearest the source sees a smaller number of rods than anotherpoint which is located at a greater distance within the interior of thepackage. 7

It can therefore be appreciated that, by virtue of a suitable choice onthe one hand of the distance at which the package is placed with respectto the source and on the other hand of the shape of the screens of thecorresponding collimation mask, the total dose received at each point ofthe package can thus be made more homogeneous, said dose being afunction of the solid angle. The prismatic screens must accordingly maskthe radioactive elements essentially in respect of those points of thepackage which are located nearest the source whereas the same prismsmust be interposed only to a very slight extent between said elementsand the center of the package and even less so between said elements andpoints located beyond the center.

Referring now to FIG. 4, it is seen that the collimation device which iscontemplated also makes it possible to improve the homogeneity of dosesreceived in the vertical direction by a package which moves in front ofthe source. In fact, any two points C and D which are spaced apart butlocated on the same vertical generator-line of that face of the packagewhich is located nearest the radioactive source, for example, do not seethe radioactive elements 2a, 2b and 2c in the same manner inasmuch aseach point is in direct line of access to radiation from only a limitednumber of elements which are either located opposite to said point orintercepted by a solid angle having its vertex at this point and being afunction of the collimation angle. Broadly speaking and in the case of aradioactive source which is constituted by linear elements such as theparallel and horizontal cylindrical rods of the embodiment of FIGS. 1and 2, the intensity of radiation received at any one point of avertical generator-line located close to the source will be determinedsolely by the radioactive elements which are located in a horizontalband. However, the width of the band must be sufficiently great topermit the intensities of radiations within said band to besubstantially equalized, while also taking into account the inherentheterogeneity of the source which arises from the fact that theradioactive rods 2 are located at a given distance from each other inthe frame 3. Moreover, the distribution of radioelements within theinterior of each rod cannot be perfectly uniform. Nevertheless, saidbands must be sufficiently narrow to limit overlapping of theradioactive source in the particular case in which the package is movedin front of this latter in only a single pass.

In order to overcome the disadvantage which has just been referred-toand which arises from inherent heterogeneity of the source asconstituted by an assembly of separate radioactive elements, an equalnumber of which is consequently not necessarily seen by two points ofthe package which are located at a distance from each other in thevertical direction, the structure of the mask 4 can advantageously bemodified by providing the screens 6 within their supports 5 with anorientation which is slightly inclined to the horizontal plane (as shownin FIG. 5) while the rods 2 remain parallel to the prisms which formsaid screens. On the other hand, the orientation of the package 7 ismaintained and, in particular, there is no modification to the directionof motion of said package in front of the collimation masks as showndiagrammatically by the arrow 8, said direction being horizontal in thiscase.

It is readily apparent that, by way of alternative, the screens andradioactive rods could be maintained horizontal while the packages aretransferred at a given angle of inclination to the horizontal it wouldeven be possible to choose two different angles of inclination on theone hand in the case of the screen and on the other hand in the case ofthe direction of transfer.

Whatever alternative embodiment is adopted, the following results may beput forward in the case of a package which has a height of 20 cm, athickness of 30 cm, a length of 50 cm, a density of 1 g/cm, which issubjected to two passes on each side of a vertically placed source andat a distance of 40 cm from said source, it is observed that the totalheterogeneity of absorbed dose which is expressed as the ratio ofmaximum to minimum doses delivered at any point of the package and whichis substantially equal to the product of partial heterogeneities in thetransverse, vertical and longitudinal directions can be evaluated at R1.40. By means of a collimation device as hereinabove described, thetotal heterogeneity of the dose is reduced under the same conditionsfrom 1.40 to 1.20 while the efficiency of the installation is reduced byonly 10 percent.

FIGS. 6 to 8 illustrate another alternative embodiment of thecollimation device in which the radioactive elements 10 are mounted in asource-holder frame 11, are of the single point type and are providedespecially in the form of spherical beads of small diameter which aredisposed on a uniform lattice having a square pitch. In this alternativeembodiment, masks 12 are associated with the source-holder frame inproximity to and on each side of the flat vertical faces of said frame,said masks being so arranged as to carry out a double collimation bothin the vertical and horizontal directions. To this end, each mask isconstituted by a grid 13 provided with a series of orifices l4 placedopposite to the single-point radioactive elements of the source and withprogressively flared recesses 15 extending outwards from said orificesin order to permit the passage of radiations. In the example of FIG. 7,said recesses 15 are designed in the form of truncated pyramids whereasin the case of the example of FIG. 8, said recesses have the shape ofcones which are also truncated. The operation of the masks otherwiseremains similar to that which was described in connection with the firstexample given above.

In consequence, and irrespective of the form of construction adopted,the judicious determination of the different parameters of the masks(angle of collimation, nature of the constituent material of said masks,distance from the source and so forth) results in substantialhomogenization of the doses received by the packages as these latterpass in front of the source without thereby imposing any unduelimitation on overall efficiency. The advantage of this device in anindustrial irradiation plant can thus take many different forms inparticular, by virtue of the use of collimation masks, the device makesit possible to achieve identical homogeneity of doses within packageswhich have the same dimensions but a different bulk density. This effectcan result in particular in enhanced efficiency of the plant inasmuch asthe increase in density substantially compensates for the relativereduction in efficiency which results from the interposition of masks.The versatility of an existing plant can therefore be substantiallyenhanced by thus increasing the range of densities of the products whichcan be processed.

The device considered also makes it possible to construct plants for theirradiation of unitary packages having a greater thickness than inconventional plants without thereby entailing any danger of substantialheterogeneities in absorbed doses and this also results in enhancedefficiency of the plant in respect of a given number of passes.

It must be understood that this invention is not limited in any sense tothe forms of construction which have been described in the foregoingwith reference to the accompanying drawings and which have been givensolely by way of example on the contrary, the invention extends to allalternative forms and can be employed in particular for the irradiationof liquid substances.

What we claim is 1. An irradiation apparatus comprising: means forgenerating radiation to be directed at a material to be irradiatedcomprising a plurality of radiation elements substantially linear in onedirection and arranged parallel to each other; and

means for masking portions of said generated radiation, including aplurality of screens made of absorbent material and being linear in saidone direction, the entirety of each screen being located between andspaced apart from said radiation generating means and said material, thetransverse cross section of each of said screens decreasing from saidradiation generating means toward said material.

2. An apparatus according to claim 1, wherein each of said radiationelements comprises a radiation generating rod and wherein each of saidscreens comprises a triangular-base prism.

3. An apparatus according to claim 2, wherein the respective faces ofsaid triangular-base prisms are substantially parallel to each other.

4. An apparatus according to claim 1, further including means for movingsaid material to be irradiated past said radiation generating means in adirection substantially parallel to a plane extending throughout saidradiation generating means.

5. An apparatus according to claim 4, wherein the direction of movementof said material is non-parallel to said one direction.

6. An irradiation apparatus comprising:

means for generating radiation to be directed at a material to beirradiated including a plurality of substantially point sources arrangedin a matrix array; and

means for masking portions of said generated radiation, including aplurality of screens made of absorbent material, the entirety of eachscreen being located between and spaced apart from said radiationgenerating means and said material, the transverse cross section of eachof said screens decreasing from said radiation generating means towardsaid material.

7. An apparatus according to claim 6, wherein each respective screencomprises an absorbent material formed in the shape of a truncatedpyramid surrounding each point source.

8. An apparatus according to claim 6, wherein each respective screencomprises an absorbent material formed in the shape of a truncatedcolumn surrounding each point source.

1. An irradiation apparatus comprising: means for generating radiationto be directed at a material to be irradiated comprising a plurality ofradiation elements substantially linear in one direction and arrangedparallel to each other; and means for masking portions of said generatedradiation, including a plurality of screens made of absorbent materialand being linear in said one direction, the entirety of each screenbeing located between and spaced apart from said radiation generatingmeans and said material, the transverse cross section of each of saidscreens decreasing from said radiation generating means toward saidmaterial.
 2. An apparatus according to claim 1, wherein each of saidradiation elements comprises a radiation generating rod and wherein eachof said screens comprises a triangular-base prism.
 3. An apparatusaccording to claim 2, wherein the respective faces of saidtriangular-base prisms are substantially parallel to each other.
 4. Anapparatus according to claim 1, further including means for moving saidmaterial to be irradiated past said radiation generating means in adirection substantially parallel to a plane extending throughout saidradiation generating means.
 5. An apparatus according to claim 4,wherein the direction of movement of said material is non-parallel tosaid one direction.
 6. An irradiation apparatus comprising: means forgenerating radiation to be directed at a material to be irradiatedincluding a plurality of substantially point sources arranged in amatrix array; and means for masking portions of said generatedradiation, including a plurality of screens made of absorbent material,the entirety of each screen being located between and spaced apart fromsaid radiation generating means and said material, the transverse croSssection of each of said screens decreasing from said radiationgenerating means toward said material.
 7. An apparatus according toclaim 6, wherein each respective screen comprises an absorbent materialformed in the shape of a truncated pyramid surrounding each pointsource.
 8. An apparatus according to claim 6, wherein each respectivescreen comprises an absorbent material formed in the shape of atruncated column surrounding each point source.